1. Field of the Invention
This invention is concerned with current sensing circuits and more particularly with a current sensing circuit used in the control circuitry of a power converter supplying balanced current to the send and return leads of a transmission line. It is specifically concerned with a current sensing circuit that responds to differential currents and not to longitudinal currents in a balanced transmission line energized by the power converter.
2. Description of the Prior Art
Longitudinal currents are often generated in transmission lines in response to magnetic coupling with other transmission or commercial power lines. The line feed converters that supply these transmission lines with balanced constant current have current control circuits that control the balanced send and return currents. The current sensing circuits used in these current control circuits typically respond to the induced longitudinal currents and attempt to regulate out its effects. These longitudinal currents, if large enough, may drive the current control circuit into saturation and shut the converter down. A suitable current control circuit for line feed converters should ignore longitudinal currents and also accurately limit the send and return currents, should either lead be shorted.
Prior art solutions to the first problem have included the use of large capacitors to bypass longitudinal currents to ground, away from the current sensing circuit. The second problem of responding to short circuits between ground and the send and/or return leads has been dealt with in the prior art by utilizing separate current control circuits for each lead or other similar types of additional current limiting circuitry.